Implantation is one of the most common techniques in the manufacturing of modern integrated circuits. In an implantation process, ions are implanted into wafers. The wafers are patterned with only desired regions exposed to the implantation, while the remaining portions of the wafers are masked. Typically, the implantations are performed by implanters, in which ions are accelerated and directed as an ion beam.
FIG. 1 schematically illustrates a portion of implanter 2 including spin wheel 10 and arms 12 attached thereon. A plurality of wafers 14 is mounted to the ends of arms 12. During an implantation process, spin wheel 10 spins at a high speed, which may be as high as 1250 rotations per minute. Accordingly, wafers 14 spin around the center of spin wheel 10. The implanter 2 further includes an opening 16, through which ion beam 18 is projected, wherein the beam current of ion beam 18 is substantially stable.
Spin wheel 10 can move back and forth in the direction of arrow 20. In the beginning of an implantation process, wafers 14, which spin at a high speed, are to the left side of beam 18. When spin wheel 10 moves right, the wafers 14 are impacted by ion beam 18. The speed that spin wheel 10 moves left and right is significantly slower than the spin speed of wafers 14. Therefore, when wafers 14 rotate by one circle, a slot on each of wafers 14 is scanned by ion beam 18. Each of the subsequent rotations of spin wheel 10 causes a new slot on each of the wafers 14 to be scanned. Typically, spin wheel 10 keeps moving right until it reaches a point that wafers 14 are on the right side of ion beam 18. At this time, the entirety of wafers 14 has been scanned slot by slot. Spin wheel 10 then starts moving left until it is back to the starting position, during which wafers 14 are scanned again. The entire process that spin wheel 10 moves once forth and once back is referred as a scan process. An implantation process may include multiple scan processes.
As is known in the art, the performance of integrated circuits depends largely upon the accuracy of the doping concentration in the implanted regions. Therefore, the dosage, or the beam current of ion beam 18 needs to be accurately controlled. To monitor the beam current, sensor 26 is placed on the projected path of ion beam 18 and at a location behind wafers 14. At a starting time and an ending time of a scan process, wafers 14 do not block ion beam 18 from sensor 26, and thus sensor 26 receives a full capacity of ion beam 18. The beam currents at the starting times and the ending times of the scan processes are thus measured to determine the stability of ion beam 18. Accordingly, dosage received by wafers 14 is monitored.
The dosage monitoring method discussed in the preceding paragraphs, however, suffers drawbacks. Since for each scan process, only the beam currents at the starting time and the end time are measured, if the beam currents drift at any time between the starting time and the end time, the drift cannot be detected. The beam current drift causes two problems. First, the total dosage received by wafers 14 will be different from the desirable value, causing performance shifts of the resulting integrated circuits. Second, different regions on wafers 14 may receive different dosages, and the uniformity of wafers 14 is adversely affected.
Between the starting time and the end time of a scan process, wafers 14 block the beam current, and the amount of ion beam 18 blocked by wafers 14 changes with the position of spin wheel 10. It is thus difficult to determine whether beam currents between the starting time and the end time of a scan process are stable or not. Accordingly, a new method for more accurately monitoring the beam currents is needed.